Food freezing is generally carried out by passing the food through the thermal zone where ice crystals of maximum diameter are formed so that the food is repidly froze. That is, the food is rapidly passed through the thermal zone of 0.degree. C.--5.degree. C. and the ice crystals of the frozen food are made smell in diameter, which avoids the cellular tissue and composition of the food being destroyed and which sufficiently suppresses the activity of microbes and enzymes.
The widespread methods of freezing food rapidly include use of a rapid freezing system such as a contact freezer, an air blast freezer etc., use of fluidized gas such as liquid nitrogen at the temperature of -80.degree. C.--90.degree. C., immersion of the food in brine and the like. Among these methods, immersing the food in brine has the advantages of relatively low facility costs and capability of mass treatment on a continuous basis.
However, the studies of the inventors of the present invention show that the rapid freezing of the food is not sufficient to minimize the change in the quality of the food while the food is being frozen. The reason is as follows. Where the food is contacted with a fluidyzed gas such as liquid nitrogen, for freezing the same, the food id rapidly frozen, with a big difference between the temperature of its outer peripheral portion and that of its core portion occurring as the freezing of the food progresses. The result is that only the outer peripheral portion of the food is rapidly frozen so that cracks occur on the outer surface of the food and the free water contained in the core portion of the food is transferred by osmotic pressure to the outer portion thereof, causing an increase in the density of liquid in the core portion so that the biosynthetic cycle may be destroyed and a change in the quality of protein can occur. Thus, the food frozen under such conditions will not restore to the original micelle when it is thawed, resulting in the deterioration of the food.
On the other hand, in the food freezing system of the type using brine, a food-transfer conveyor disposed in the brine is utilized for immersing the foods in the brine continuously.
This type of system has rotary portions, some of which are exposed above the brine and are frozen by the same, and thus the conveyor is difficult to drive continuously. The conventional attempts in solving the problem include heating the rotary portion of the conveyor exposed above the brine by means of a heater and providing a heat-insulating layer (member) between the rotary portion and the remaining portion of the conveyor immersed in the brine. However, the former attempt increases the cost and the latter one is not sufficient to protect the rotary portion from freezing.